1. Field of the Invention
The present invention relates to a high electron mobility device having a two-dimensional electron gas (2DEG) formed at a hetero-interface of different polytypes of a semiconductor material, and the method of making such a high electron mobility device.
2. Background
High electron mobility transistors (HEMTs) demonstrate the ability to operate at very high frequencies. By using two different materials having different respective bandgaps, charge may be confined to the interface between the two different materials to thus form a two-dimensional electron gas (2DEG) that realizes high mobility in the direction of current flow along the 2DEG. Typically, a HEMT structure includes a 2DEG formed at the interface between an indium gallium arsenide layer and a gallium arsenide layer, a so called InGaAs/GaAs system. Another example of a typical HEMT structure includes a 2DEG formed at the interface between a gallium arsenide layer and an aluminum gallium arsenide layer, a so called GaAs/AIGaAs system.
Silicon carbide (SiC) is an attractive material for high-frequency power applications. However, in the case of silicon carbide, it is not possible to form a ternary alloy that allows the bandgap of the material to be engineered or changed in order to create a SiC heterojunction with a band-offset in either the conduction or valence band. In other words, there are no elements that can conveniently be added to SiC to change the bandgap of the material in a suitable manner as necessary to form a 2DEG.
In SiC, the bandgap of the material is dependent upon the crystal type in which the atoms are arranged. By combining two crystal polytypes, a heterojunction can be formed at the interface between the two. However, the growth of two polytypes in contact, whereby both crystals are of high quality and form a single domain, is a significant challenge. Accordingly, there is a need to provide a SiC high electron mobility device, and in general to provide high electron mobility devices for materials that have no suitable ternary alloys, whereby high quality crystal polytypes are used to form a hetero-junction.